Power unit



Dec. 31, 1957 F. P. GERHARDT 2,318,054

POWER UNIT Filed Deo. 28, 1956 I 3 Sheets-Sheet 1 Dec. 31, 1957 F. P.GERHARDT 8.18.5054

POWER UNIT Filed Dec. 28, 1956 3 Sheets-Shee't 2 w Qn *W Dec. 3l, 1957F. P. GERHARDT POWER UNIT 5 Sheets-@JheefI 3 Filed D90. 28, 1956 c ONMOooh oohu cova oom OQ OE United States Patent PWR Fred l. Gerhardt,Nierstein, (Rhine), Germany Application December 28, 1956, Serial No.631,238

15 Claims. j(Cl. 12S- 191) This invention relates to lnew and usefulimprovements in power units or prime movers and in general is animprovement on the power unit coveredhby my prior United States PatentNo. 2,534,590, dated YDecember 19, 1950.

The general objects of `the invention are to provide a power unit of theenclosed type wherein all the necessary events for a yf ull and completeoperation are carried out within a cylinder; and to provide a power unitdriven by internal combustion which hasta greatly increased efiiciencyand power without increasing the maximum pressures and temperaturesnormally found within a single cylinder unit with comparativecompression ratios.

As such, the invention facilitates conversion into power a largeproportion of the he'at ordinarily -lo'st in the cooling system andexhaust o'f conventional power units. The unit in accordance with theinvention is equally well adapted for burning of fuel' by spark ignition(constant volume) or by compression ignition (constant pressure).

The general operation and design of the invention is based on threemajor and well known premises: the theorem of Johann Bernoulli (1728);the experimental findings and calculations by Prof. 'George Wilson ofCambridge (1875); and since then the 'development of the de Lavalnozzle. To the latter may be added the action within a dilfusor which islargely like that of a nozzle in reverse. These basic factors are allweil known, have been thoroughly proven, and are in general use daily.They have been particularly tested and formulated by Prof. AurelStodola.

The two basic laws lof thermodynamics havey been followed throughout, asexpounded by well known authorities such as Carnot, Clausius, Thompson,Joule, Stodola, and others. These basic laws have been found to be trueand universal and unchanged even by Einsteins findings and the theory ofrelativity.

Prof. George Wilson found by experiment and calculan tion that: I-fsteam be allowed to expand behind a piston in a cylinder from P1 to 0.57P1 adiabatically, the mean effective pressure will be about 0.33 P1. Ifa stream capable of exerting this mean pressure were allowed to owthrough an orifice, it would be able, according to the principlesgoverning the impulse of jets of fluid, to exert an impulsive pressureand, therefore, a reaction 'of twice the pressure corresponding to itsstatic head, or 0.66 P1. Besides this pressure the reaction would beincreased by the addition of the pressure in the orifice, or 0.57'P1absolute, the atmospheric pressure must be subtracted. The expressionfor the reaction then becomes:

:1.23 P1-14-71bs. per sq. in. of orifice :1.23 P1- 14.7 lbs. per sq. in.of orice The gases of combustion, mostly air, follow the samethermodynamic `laws as steam when expanding or contracting, but P1becomes 0.527 P1 for air, in the results given above. o

These findings were and-still are very importantfacts which form thebasis of designs Iby Parsons and other turbine builders. However, steamand gases of com` bustion are two very dilferent power vehicles whichhave diiferent characteristics even though they follow the samethermodynamic laws generally.y These differences in characteristicsdemand different handling with different designs and construction of thenecessary means to produce equal and comparatively useful results. Thishas been borne out by developments in the gas turbine field as comparedto the steam turbine field, as known today. The gas turbine strictly asa turbine similar to the steam turbine has lagged far behind the steamturbine because the different characteristics and differences invhandling gases of combustion instead of steam have not been usedproperly. The great temperature dilferences alone are primary factorswhich undeniably -call for primary diiferences in means if the same orequal results are to be expected.

If a metal were found which could withstand the high temperaturesinvolved, or a fuel with the necessary characteristics to enable a givencombustible gas to replace steam within a standard turbine and performequally well therein, a large air compressor and a large furnace as Wellas many additional stages of expansion would be required. The resultantpower plant for equal power would be practically as large and cumbersomeand dificult to maneuver as the present steam turbine plant and theoverall efficiency would not be much greater, if at all.

In the present invention, all 'the factors and means included accordingto the la'ws of thermodynamics and mechanics indicate that the resultsobtained by Prof. George Wilson are attainable with gases within apiston and cylinder engine in a simple, practical and useful manner.Moreover, the power unit can be safely used in sizes large enough tocompete with the largest steam turbine plants, very successfully as toboth power and efficiency, but be very superior lin maneuverability. andadaptation to many uses for which the turbine, strictly as such, cannotbe used.

The present invention will be understood by reference to theaccompanying drawings, wherein like characters of reference are used todesignate like parts, and wherein:

Figure 1 is a vertical sectional view of a power unit in accordance withthe invention;

Figure 2 is a fragmentary sectional view, taken substantially in theplane of the line 2-2 in Figure l;

Figure 3 is a cross-sectional view, taken substantially in the plane ofthe line 3 3 in Figure 2; f

v Figure 4 is a fragmentary `sectional detail of one of the passages;

Figure 5 is a fragmentary vertical sectional view illustrating amodified form of the invention;

Figure 6 is a vertical sectional view of another modified form; and

Figures 7, 8 and 9 are graphical illustrations of some of the factorspertaining to the operation of the invention.

With particular reference to Figures 1 4 inclusive, the numeral 10designates a cylinder of the power unit, the same being provided with anexternal head 11 which is -secured thereto by suitable bolts 12.Disposed within the cylinder 10 'and secured to the external head 11 bysuitable bolts 13 is an internal head 14 whichvcoacts in a mannerherein'after'described with 'a reciprocable piston 15, equipped with theusual vwrist pin 16 and connecting rod 17. The piston is also providedwit'hthe usual set of rings 18, while the external head '11 is providedwith igniting means such as a spark plug 19. Suitably actuated inlet andoutlet valves may be 'also provided in the head 11, as indicated at Z0,21, respectively.

The internal head 14 is recessed to povidea 'central combustion chamber22 substantially of the configuration shown, and `is also provided withan Aannular recess as, coaxial with the l,cham-ber a2. The 'temarios nfthe chamber and recess 23 results in the provision of a set of annular,coaxial projections 14a, 14h on the head 14, which projections arereceivable in complemental recesses 24, 25, respectively, formed in thepiston 15. The formation of the recesses 24, 25 in the piston, in turn,results in the provision of an annular projection 15o on the piston,which is receivable in the complemental recess 23 of the head 14. n

The projections 14a, 14b and 15a t into the respective recesses 24, 25and 23 with sutlicient clearance to form a torturous passage 26 whichextends from the lower end of the chamber 22 between the interttingprojections of the piston and head 14 to the side wall of the cylinderand extends therealong to the external head 11, at which point thepassage 26 communicates with the upper end of the chamber 22 through aspace 26a existing between the heads 11, 14. The drawings, of course,illustrate the piston at the top of its stroke, in which position theparts are interiitted as shown. As the piston travels downwardly, theprojections 14a, 14h, 15a become withdrawn from the respective recesses24, 25, 23, so that the volume of the passage 26 is substantiallyenlarged, although the volume of the chamber 22 remains the same.

The invention may be embodied in a four-cycle or a two-cycle engine, andthe engine may be supercharged, if desired, in a conventional manner.

The engine in accordance with the invention is far more eflicient thanconventional engines as indicated by Prof. Wilsons experiments, and thepower difference is still greater because of the greatly increased M. E.P. These differences are particularly achieved because of the method ofenergy flow utilized, and the structure represents simple and practicalmeans of converting the energy ow into impulsive pressure and reaction,and therefore Work on a moving piston exactly along the lines propoundedby Prof. Stodola in his mathematical analysis and proof as developed inhis theorem of Bernoulli. Therein he proves that within a given time ina given energy ow a reduction of energy between two crosssections isreected equally in the two cross-sections. In order to keep this owcontinuous and in the same direction until the energy is used up, theremust be a minimum (only one) cross-section with an expanding nozzle, andbehind this a larger cross-section which forms the throat of acompression nozzle or difusor which keeps on sup- A plying the remainingor major part of the energy upon its return from the piston and lowerhead (internal head 14).

The minimum expansion nozzle is disposed in the chamber 22 at 27 and thesmall end of the difusor is shown at 28. The space or portion of thechamber 22 between the regions 27, 28 is the combustion and compressionchamber proper. The highest relative pressure of the system is alwayswithin the chamber 22 during the expansion stroke and therefore, theenergy ow is continuous until it is all used up. Thermodynamically, theisothermal system finds existence in the chamber 22 as its only place;as the gas ows through the nozzle 27 its state changes from theisothermal to the adiabatic system, expanding through the nozzle and theWork well between the piston and the internal head 14 where the pressureis at its lowest. From that point of lowest pressure the design becomesa diffusor and the remaining or major part of the momentum of the gassesis gradually destroyed as it is changed to pressure and reaches itsmaximum relative pressure again within the chamber 22 after flowingthrough the section 28. The expansion side forms an adiabatic. thediifusor or compression side the other adiabatic, while the work well islocated between the two. There is, therefore, only one isothermal andthat is located in the chamber 22.

It is evident from the foregoing that this is an enclosed system fromthe beginning of the working stroke when the ow of energy is started byignition and as long as the flow of energy continues, to the end of thestroke when the gasses are exhausted in the usual manner. .Each elementof the gas at it returns to the chamber 22 1s, of course, at a lowerpressure after each trip throughthe work well, since there, a portion ofits energy 1s given up as work against the piston. The piston is movingand the work well is increasing continually in volume, but the chamber22 and most of the compression space is not. The size of the nozzle 27is such that the pressure in the chamber 22 is always considerablygreater than that in the nozzle 27. When velocity of gases above that oflocal speed of sound is required, then the ratio of pressures betweenthe chamber 22 and the nozzle 27 must be as l is to 0.527. Since this isthe highest ratio found in any cross-section of the system, the energyand the gas flow will be continuously in the same direction andcontinuously go through the events as above described until all ornearly all of the available energy is converted into work.

As will be apparent, friction and resultant heat of friction will exist,but in this design practically all the heat of friction is convertedinto work because each element of gas upon its return to the chamber 22returns each time, or is transformed each time it returns back into theisothermal system which means that all the heat content of each elementjoins up each time with all the rest of the heat in the chamber 22 andhelps build up and maintain the maximum relative pressure desired. Italso forms part of the continuous energy ow which then flows through thenozzle 27 as the isothermal system changes to the adiabatic in or nearthe throat of the nozzle 27.

This design is therefore practical and workable in construction which isin a large measure similar to the improved theorem of Bernoulli. Thereis however one additional or practical feature, which is the work well.This and other practical dilerences exclude any attempt at perpetualmotion. At this point a quotation from Osbomes Marine Engineers Manualmay well be taken, which not only explains the theory of thermodynamics,but also practical working out and use thereof in engines. Because ofthe many different conditions under which steam (and gas) may be causedto expand in performing actual work, the factors of expansion arevariable quantities. These are known as adiabatic and isothermalexpansion and are of great importance in the theory of per formance ofheat engines.

Adz'abazic expansion-A gas is said to expand or be compressedadiabatically when it does not receive or give out heat, except in theform of external work. That is. there is no radiation or conduction ofheat to or from the gas, as it is expanded or compressed in doing actualworlz. In other words, any external work done by the gas is done at theexpense of the contained or internal energy of thc gas. All the heat andinternal energy lost during expansion of the gas goes to doing work. Ifcompressed adiabatically, all the work done on the gas goes to increaseits internal energy. Since any change in the internal energy of a gasdepends upon a change in temperature, it is impossible to have anincrease in the internal encrg f without an increase in temperature, ora decrease in internal energy without a decrease in temperature. lnactual heat engines or compressors this is never true, and the adizebatic expansion o1' compression is only approximate. This theory ofadiabatic expansion, however, serves in engineering calculations as anideal or standard for comparing the actual performances of heat engines.

Isothermtzl'expansion-The term isothermal means equal heat. Isothermalexpansion then means expansion at equal or constant temperature. A gasexpands or con tracts isothermally when its temperature remains constantduring a change of volume. That is, in isothermal expz=nsion orcompression, a gas Amay be expanded from a high to a lower pressure, orit may be compressed from a larger to a smaller volume; but in doingeither it must absorb or give off enough heat to keep its temperatureconstant."

Pi''tl'l 'exl'mplels' 0f lidl'bdbai Lind Sfhel lvSt-- The foregoing maybe better explained fas follows. When a gas expands in a cylinder iundera movable piston and the piston is moved by sornegyflorce appliedexternally, then the volume and the pressure of the enclosed gas wouldchange but the temperature would remaihconstant prof viding there wereno losses due to radiation, etc. Therefore, the curve indicating thevarying condition of the gas as to pressure and volume would be anisothermal curve. If however, .the piston were moved by expenditure ofthe heat energy -stor'ed ih 'the gas, -their extferial work `would bedene ey 'the gas and heat energy expended. The pressure wnulci rau tobelow that af the isetue'rmai curva. Then, if no other heat influenceshad been roduced, such as heat by conduction through the cylinder wallor gain of heat from some external source, the curve de scribed would bean adiabatic curve.

Thia improvements Possible, will? this maths@ and dign can also be nos@,mamma ,any .au yh any. the@ proofs coinciding closely with the resul dlah@ dt?- monstrated by Prof. Wilson, Mr. Rosenhaih, 'and others. Suchfurther proofs are as follows: The process herein described and of whichthis yinvention is a particular example 0f Construction has beenCalculated, by Prof. S19* dola, Poincare and other authorities onthermodynamics. Stodola used a theoretical construction in hiscalculations to prove that the second law of thermodynamics was soundand unassailable even by the theory of relativity. (It can be found inStodolas treatise on Steam and Gas Turbines, 6th edition, 1924. 'Forexample, Stodola found and developed formulae f or isothermal andadiabatic ow of steam 'or gases which are true and now in practical use.Flaute, 7, f the accompanying drawings Shows a translation of the mannerin which an energy flow of steam or 'gas Igoes through 'a series pfnarrowed down 'crossscctions It is important to observe that thecritical pressure lim does not occur in every small cross-section, butonly in the smallest or narrowest cross-section such as fm(corresponding tothe nozzle 2,7) since there in the relation G=fqf`(-p')the factor qrp) must have the 'greatest 'possible value. G is the'weigh-'t of new, q is the velocity factor and 'p is the pressure. l'nthe two narroweddown "sections fm-fm, the function q( p) starting from Aand mo'vin'g toward fm will increase, but not to its highest value, theother side of fm the Value q'Q'p) goes through also p with the sainevalues as before, but fp is reversed in vdirection. In B where f is amaximum, yfunction q(p) will be the smallest (without being analyticallya minimum), the pressure has a maximum value. From B toward fm thepressure reduces, and when .fitr-cfm, we also have the` same px asbefore. Beyond fm however, an expansion follows and when f=],E again,then'q(p"')=q(px)1. On the other hand the pressure itselfcorresponds toa value less than pX at a point the other side of the maximum fqtp).Were fm'=f Ithe pressures at both places would 'be the same.

The formula G,ec=f,q(-p,) represents the highest value of GsEc which can`ilow through a given cross-'section under a given set of pressureconditions. A weightfofilow where G"SE,C Gsec is Very possible. In this`case the pressure becomes less toward fm, q(p-) increases butwithoutreaching the highest value at jm. After this the 'saure values ofq(-'p) and of p which took 'place before fm must 'run through, jt-hatis, the `steam or gas on the after or other side of the narrowestcross-'section will again become compressed, and vwill theoreticallywith sufficient widening 'of the Anozzle reach the same value that ithad in the beginor original chamber.

Cnsequenny, Such a new when ence 'ser in motion, aside from the lossesdue to friction, etc., would continuously go around such 'a circuit.However, power losses take place therein as in 'any other vengine and itwould soon stop unless additional energy were not continually suppliedto keep it moving.

In the invention hot gases may be made to convert heat energy into workby impulse and reaction against a piston, reaction taking place alongwith impulse when the velocity of ilow is greater than the local speedof sound.

With reference again to Figures l-4 of the drawings, it will be notedthat although the nozzle 27 is disposed at the outlet of the chamber 22,the walls of the entire passage 26 form a continuous expanding nozzle ingeneral which is a continuation of the nozzle 27 and operates insubstantially the same manner until the gases reach the head space 26a.This is made possible by the fact that the ilow of gases in general isradially outwardly from the center of the cylinder as well as axiallyalong the 'direction of stroke. When the gases reach the outer wall ofthe cylinder 11 and the head space 26a they ow radially inwardly to thedilfusor section 28. The remaining kinetic energy at the circumferenceof the space 26a along with the inner energy or heat and also the -major.part of the heat of friction caused by the flow, combine and uniteincontinuing the ilow of gases into the chamber 22. Here the gas mediumwith its various energy components changes back into the isothermalsystem or state. During this action the pressure in the chamber 22 goesdown continually because of the work done on the piston and because ofsome conduction and radiation losses. This reduction in pressure withinthe chamber 22 therefore follows lthe isothermal expansion curve Veryclosely, but of course does not completely equal it. t

The lrelative sizes of the nozzle 27, section 28, etc. are such thatfrom Athe beginning to the end ofthe stroke the velocity and weight yofgases flowing continuously will provide the right amount of impulse and'reaction for a given engine.

It willvbe also observed lthat the aforementioned projections 14a, 14band 15a have rounded extremities 29 and that the recesses 24, 25 and 23have rounded bottom edges 30, the curvatures of the respectiveextremities 29 and edges 30 being of such diameters and spaced in suchmanner as to produce restricted regions 31 as is best shown in Figure 4.In these restricted regions of the passage 26 the gas will be somewhatcompressed so that its speed is that Vof sound. The friction loss atsuch speed is low, since it is turned to heat which further assistsexpansion and is practically all converted to work.

There are great many phenomena in nature who'se rate of increase isproportional to itself and the only known function which shows thevariations is the exponential function where y=e, wherein thedifferential coe'icient of eX is eX itself. The isothermal state anditsy change in a nozzle throat to an adiabatic state presents an energycontent whose possibility to do work is shown by the well known formulaEntropy diagrams can also be constructed and the various heat and workcontents compared to obtain a correct nozzle size. These, however, areAcumbersome and hard to grasp. This particular invention however, lendsitself to a simple graphical analysis and also gives a more completepicture which can serve well in calculating the various sizes ofnozzles, etc. for engines of this type. In this analysis a simple gasengine indicator diagram is used as follows:

It can be proved that when a perfect gas (whose law is pv=Rt for a poundof gas, R being constantl and equal to K--k the difference of theimportant specific heats; & is used to denote K/k) changes in its volumeand pressure in any way, the rate of reception :of heat (or release ofheat) by it per unit change of volume, which may be called h (in workunits) or sumed to be expressed in work units so as to avoid theunnecessary introduction of J' for Joules equivalent. When gas expandsaccording to the law Pvs=c (3) a constant,

& '#558111 4) Evidently, when s=&, h=0, and therefore we havepv&=constant as the adiabatic low of expansion of a perfect gas. & is1.41 for air and 1.31 for the particular gas in the engine cylinder.When s=l, so that the law of expansion is pv constant, we have theisothermal expansion of a gas and we notice that here h=p, or the rate`of reception (or giving up) of heat energy is equal to the rate `of thedoing of mechanical energy. It can be seen that in any case where thelaw of change is given by Formula 3, h is exactly proportional to p. Ifsis greater than &, the gas is having heat withdrawn from it. If theEquation l be integrated with regard to v, we have Here H is the heatgiven to a pound of perfect gas between the states p0, v0, to and p1,v1, t1 and W is the work done by it in expanding from the first to thesecond state. This expression may be put in other forms because we havethe connection afk@ where k is a constant, being the specific heat atconstant volume. Integrating this with regard to v, we find This givesus exactly the same answer as the last method and may at once be derivedfrom (5) and (6). In this form one sees that if no work is done, theheat given is Mtl-t0) and also that if there is no change oftemperature, the heat given is equal to the work done. In applying theabove, two diagrams were made for h, one for the adiabatic expansion andone for the isothermal (the isothermal expansion curve here is hitself), using s=l.31, a stroke of 3.37 inches and a compression ratioof 7 to 1, as shown in Figure 9. h is the rate at which the gas showsthat it is receiving heat in foot-pounds per unit change of volume, onthe assumption that is a perfect gas receiving heat from some furnace.(In fact, it

is its own furnace, the heat coming from its own chemi cal energy.) Justas pressure is the rate at which work is done per unit change of volume,so is dH bra;

It is to be observed that h is in the same units as p, and to draw acurve for h itis not necessary to pay any attention to the scales foreither p or v. They may be meas ured as inches on the diagram. Usingthese values and having found h or at every place, and it is desired tofind the rate per second at which the gas is receiving (or giving off)heat, if t represents time,

einer a dt du dt and so it is only necessary to multiply h by stant Thisis shown in Figure 9, the curve dlill being for adiabatic and the curvedHIx for isothermal.

As will be observed, the slope of each is practically equal to itsordinate as in exponential curves in general. The area under each showsthe energy available in footpounds and their comparative ratio. Thisratio is also shown by the slope of these curves. The curves clearlyshow that although the maximum pressures are the same, the workavailable and therefore the mean elective pressure is much greater withthe present invention. They also show that the pressure on the pistonreduces in a straight line ratio which means that in a piston andcylinder engine, a given sized nozzle may be calculated from the usualformulae used to tit the requirements of a given engine in order to `useup most of the available energy during the length of the stroke. Thereis a steady and even reduction of pressure from the beginning to the endof the stroke.

In the diagram shown in Figure 8, the curve A, according toMr.Roserihains experiments, indicates velocity corresponding to measuredreaction of the jet from an expanding nozzle, while curve B representscalculated velocity, assuming that all the heat energy concerned in thedrop from the higher pressures before the nozzle to the constantatmospheric pressure beyond was converted into kinetic energy of the jetstream. Y is percentage loss in efficiency.

Referring now to the accompanying Figure which shows a modifiedarrangement of the invention wherein the external head 11 is providedwith a depression 31 having the igniting means 19 mounted therein, thedepression 31 coacting with the internal head 14 to provide the diffuserregion 28a adjacent the center of the space 26a in place of the region28 of Figure 1. The total cross-section of the region 28a is greaterthan the crosssection of the Anozzle 27 and, as in Figure l, thecrosssections in the passage 26 are greater than the nozzle, so that thenozzle is the only minimum cross-section in the gas flow.

Figure 6 illustrates another modied embodiment of the invention whereinthe cylinder 32 is provided with an external head 33 which is securedthereto by suitable screws 34, while an internal head 35 is secured tothe head 33 by suitable screws or bolts 36. The igniting means are shownat 37, while 38 is the piston reciprocable in the cylinder. The pistonis provided with a wrist pin 39 and with a set of rings 40.

In this form of the invention the aforementioned torturous passage 26,26a of Figures 1 and 5 is substituted by a comparatively simple passage41 which extends from the nozzle 42 of the combustion chamber 43 in thehead 35 downwardly, radially outwardly and then upwardly between curvedprojecting bottom portions 35a of the internal head 35 andcomplementally curved recesses 38a formed in the piston 38, as will beclearly apparent. The passage 41 then continues upwardly and radiallyinwardly through a space 44 between the heads 33, 35 to a diffuserregion 45 at the top of the chamber 43. As will be apparent, this formof the invention is considerably simpler than the forms of Figures l and5, but the operating characteristics thereof are substantially the same.

While in the foregoing there have been described and shown the preferredembodiments of the invention, various modifications may become apparentto those skilled in the art to which the invention relates. Accordingly,it is not desired to limit the invention to this disclosure, and variousmodifications may be resorted to, such as may lie within the spirit andscope of the appended claims.

What is claimed as new is:

1. In a gas expansion engine, the combination of a cylinder, a headthereon provided with a combustion chamber, a piston reciprocable insaid cylinder and spaced from said head at the top of its stroke toprovide an expansible gas passage therebetween in communication withsaid chamber, and means for returning gases from said passage throughsaid head to said chamber.

2. The engine as defined in claim l wherein said chamber is of a fixedvolume.

3. The engine as defined in claim l together with a restricted nozzleprovided in said head between said chamber and said passage.

4. In a gas expansion engine, the combination of a cylinder, a headthereon, and a piston reciprocable in said cylinder, said head beingprovided with a combustion chamber and with a gas return passagecommunicating with said chamber, said piston at the top of its strokebeing spaced from said head to provide an expansible gas passagecommunicating with said chamber and with said return passage, wherebygases expanding in said expansion passage may be returned to saidchamber.

5. The engine as defined in claim 4 wherein said head is provided with arestricted nozzle between said chamber and said expansible passage.

6. The engine as defined in claim 4 wherein said head is provided with agas dilusor section between said return passage and said chamber.

7. The engine as defined in claim 4 wherein said chamber is of a fixedvolume.

8. In a gas expansion engine, the combination of a cylinder, an externalhead provided thereon, an internal head secured to said external head insaid cylinder and provided with an axially elongated combustion chamberof a fixed volume having a restricted nozzle at its lower end, saidinternal head being spaced from said external head to provide a returngas passage therebetween in communication with the top of said chamber,and a piston reciprocable in said cylinder and spaced at the top of itsstroke from said internal head to provide an expansible gas passagecommunicating with said return passage and with said nozzle, wherebygases expanding in said expansible passage may be returned to saidchamber.

9. The engine as defined in claim 8 wherein the spacing between theexternal and internal heads provides a gas diffuser section between saidreturn passage and said chamber.

l0. The engine as defined in claim 8 together with at least one annularprojection provided on said internal head and having an annular recessadjacent thereto, said piston having an annular recess receiving saidprojection, and an annular projection provided on said piston adjacentthe recess therein and received in the recess in said internal head,said projections on said internal head and piston being spaced apart toconstitute said expansible gas passage.

11. The engine as defined in claim l0 wherein said projections haverounded extremities and wherein the recesses in said internal head andpiston are complementally rounded.

12. In a gas expansion engine, the combination of a cylinder, anexternal head provided thereon, an internal head secured to saidexternal head in said cylinder and provided with an axially elongatedcombustion chamber of a fixed volume having a restricted nozzle at itslower end, said internal head being spaced from said external head toprovide a return gas passage therebetween in communication with the topof said chamber, a piston reciprocable in said cylinder, a set ofcircumferentially spaced annular projections provided on said internalhead, and an annular projection provided on said piston and received inthe space between the projections of said internal head, said pistonbeing spaced at the top of its stroke from the internal head to providea torturous expansible gas passage between said projections, saidexpansible passage communicating with said return passage and with saidnozzle, whereby gases expanding in said expansible passage may bereturned to said chamber.

13. The engine as defined in claim 12 wherein said nozzle affords thesmallest cross-section of gas ow in the engine.

14. In a gas expansion engine, the combination of a cylinder, anexternal head provided thereon, an internal head secured to saidexternal head in said cylinder and provided wit/h an axially elongatedcombustion chamber of a fixed volume having a restricted nozzle at itslower end, said internal head being spaced from said external head toprovide a return gas passage therebetween in communication with the topof said chamber, a piston reciprocable in said cylinder, and an annularprojection provided on said internal head, said piston being providedwith an annular recess receiving said projection, the piston beingspaced at the top of its stroke from said internal head to provide anexpansible gas passage at said projection in said recess, saidexpansible passage communicating with said return passage and with saidnozzle, whereby gases expanding in said expansible passage may bereturned to said chamber.

15. The engine as dened in claim 14 wherein said nozzle affords thesmallest cross-section of gas iow in the engine.

No references cited.

